1. Field of the Invention
The present invention relates to a dielectric filter, a dielectric duplexer, and a communication device.
2. Description of the Related Art
A known dielectric filter in which a plurality of dielectric resonators are provided in a dielectric block is shown in FIG. 20. The dielectric filter 200 is formed in a dielectric block 201 having a generally parallelepiped shape. A pair of resonator holes 202a and 202b are formed in the dielectric block, each hole extending between opposing surfaces 200a and 200b of the dielectric block. The resonator holes 202a and 202b have large-diameter hole portions 222a and 222b, and small-diameter hole portions 223a and 223b communicating with the large-diameter hole portions 222a and 222b, respectively.
As best shown in FIG. 21, the end walls 224a and 224b of the large-diameter hole portions 222a and 222b and end walls 225a and 225b of the small-diameter hole portions 223a and 223b are formed in a common plane. The axes of the small diameter hole portions 223a and 223b are displaced from those of the large diameter hole portions 222a and 222b with the result that relatively small communication areas b are formed between the respective large and small diameter hole portions.
An outer conductor 204 is formed on five of the six outer surfaces of the dielectric block. The front surface 200a is not plated. A pair of input/output electrodes 205 are formed on the outer surface of the dielectric block 201 and are spaced from the outer conductor 204 so as to be electrically isolated therefrom. Inner conductors 203 are formed on the entire inner surface of each of the resonator holes 202a and 202b. The end of the inner conductors 203 located at the front surface 200a of the dielectric block is electrically open (i.e., spaced from, and thereby isolated from, the outer conductor 204). The end of the inner conductors 203 located at the rear surface 200b is short-circuited (physically connected) to the outer conductor 204.
The outer conductor 204 and inner conductors 203 are typically formed on the dielectric block 201 by wet plating. However, with wet plating, the plating liquid in the vicinity of a surface to be plated must be circulated so that new plating liquid is constantly supplied to the surface. To this end, plating liquid is typically stirred or the workplace is moved in the plating liquid to promote the circulation of the plating liquid.
As best shown in FIG. 21, the connection portions b between the large and small diameter portions are narrow. This results in poor penetration of the plating liquid through the resonator holes 202a and 202b, and thus results in a smaller supply of new plating liquid and insufficient plating. With this arrangement, therefore, it is difficult to provide the desired film thickness for the inner conductor 203 to be formed on the inner surface of the resonator holes 202a and 202b. 